Method for producing single electron semiconductor element

ABSTRACT

The present invention provides a method for production of a single electron semiconductor element (SET) in which a quantum dot is selectively arranged in a nano gap between fine electrodes, whereby the product yield is significantly improved, leading to excellent practical applicability. The method for production of SET of the present invention is characterized in that a solution containing ferritin including a metal or semiconductor particle therein, and a nonionic surfactant is dropped on a substrate having a source electrode and a drain electrode formed by laminating a titanium film and a film of a metal other than titanium, whereby the ferritin is selectively arranged in a nano gap between the source electrode/drain electrode.

This is a continuation application under U.S.C 111(a) of pending priorInternational application No. PCT/JP2006/323730, filed on Nov. 28, 2006,which in turn claims the benefit of Japanese Application No. 2006-028438filed on Feb. 6, 2006, the disclosures of which Application areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method for production of a singleelectron semiconductor element, in particular, relates to a method forproduction of a single electron semiconductor element in which a nanodot (quantum dot) composed of a metal or a semiconductor is arrangedbetween a source electrode and a drain electrode.

BACKGROUND ART

In order to increase the degree of integration of semiconductor memoryelements, development of novel techniques in production steps has beendemanded. When a MOS structure is utilized in semiconductor memoryelements, highly integrated semiconductor memory elements such as DRAMof approximately 4 giga, a gap between the source electrode and thedrain electrode becomes as small as approximately 0.13 μm. Therefore, itis predicted that switching by gate voltage which has beenconventionally utilized as an operation principle of MOS elements isdisabled. Hence, small gap between the source electrode and the drainelectrode causes malfunctions of the elements may be caused due tophenomena of tunneling through the gate oxide film, and of tunnelingbetween the source electrode and the drain electrode even in the statewithout applying the gate voltage. Therefore, for manufacture ofgiga-order or tera-order devices, it is necessary to use modes otherthan those currently used on the basis of the MOS structure.

As one of the elements of such novel modes, single electronsemiconductor elements (also referred to as “Single ElectronTransistor”, hereinafter, abbreviated as SET) in which a single electrontunnel effect is utilized have been focusing attention.

SET is an element which works on the basis of a phenomenon referred toas coulomb blockade. More specifically, elements that operate dependingon alteration of electrostatic energy generated upon tunneling of oneelectron as a unit between fine conductors which can be charged ordischarged are referred to as single electron element. Operation of thesame has been confirmed in the form of single electron memories, singleelectron transistors and the like. Junctions which can result inobservation of such a coulomb blockade phenomenon are referred to asfine tunnel junctions.

In conventional SET, fine tunnel junctions have been formed by finepatterning with electron beam lithography (for example, see NonpatentDocument 1 (T. A. Fulton and G. J. Dolan: “Observation ofSingle-Electron Charging Effects in Small Tunnel Junctions”, Phys. Rev.Lett. Vol. 59, No. 1, pp. 109-112 (1987).), however, capacity of thefine tunnel junction which is formable thereby can not make smallenough, therefore, operation of SET at a room temperature was difficult.As a matter of fact, for enabling the observation of such a phenomenonreferred to as coulomb blockade as described above at a roomtemperature, alteration of the electrostatic energy should besignificantly large as compared with the alteration of thermal energy.

To this end, size of the conductor which can be charged or dischargedshould be equal to or less than 20 nm, in addition, intervals providedby arranging these conductor which can be charged or discharged shouldbe equal to or less than several nanometers. Such a minute fine tunneljunction can be hardly produced by a pattern formation method accordingto currently employed lithographic technique. Alternatively, even if itcan be produced, production of large quantity in a high yield isextremely difficult.

As a method for production of the SET structural device by arrangingnano particles between fine electrodes formed on a substrate, NonpatentDocument 2 (A. Dutta et al.,: “Single-Electron Tunneling Devices Basedon Silicon Quantum Dots Fabricated by Plasma Processes”, Jpn. J. Appl.Phys. Vol. 39, pp. 264-267 (2000).) discloses a method in which Si nanoparticles are fixed on a substrate on which fine electrodes (sourceelectrode and drain electrode) are provided, and an Si particle chain isformed in a nano gap between the source electrode and the drainelectrode (hereinafter, referred to as nano gap between source/drain).

Also, Nonpatent Document 3 (T. Sato et al.,: “Sigle electron transistorusing molecularly linked gold colloidal particle chain”, J. Appl. Phys.82 (2), p 696 (1997).) discloses a method in which an Au nano particlechain is formed in the nano gap between source/drain by allowing Au nanoparticles to be adsorbed on a substrate on which fine electrodes (sourceelectrode and drain electrode) are formed, then modifying the Au nanoparticle with dithiol, and further allowing for adsorption of the Aunano particle.

DISCLOSURE OF THE INVENTION

Problems that the Invention is to Solve

However, according to the method disclosed in Nonpatent Document 2 orNonpatent Document 3, it is difficult to arrange the nano particlesselectively in the nano gap between the source electrode and the drainelectrode, and it merely utilizes as a quantum dot the nano particlethat happened to be arranged in the nano gap.

In other words, according to the method disclosed in Nonpatent Document2 or Nonpatent Document 3, a large number of quantum dots are formed atrandom on the substrate on which the fine electrodes are formed.Therefore, as shown in FIG. 14A, it is impossible to regulate thedistance between the quantum dot 14 and the fine electrodes 5 and 6(hereinafter, gap between quantum dot/fine electrodes), and also it isdifficult to utilize the gap between quantum dot 14/fine electrodes 5and 6 as a tunnel barrier.

Furthermore, as shown in FIG. 14B, even though the quantum dot is formedby accident at a center location of the nano gap, many unwanted quantumdots 15 can be present around the nano gap. Therefore, there also ariseproblems of alteration of the operation point of SET by the chargetrapped in the unwanted quantum dots.

Therefore, according to the method disclosed in Nonpatent Document 2 orNonpatent Document 3, just ones in which the quantum dot was formed onlyin the nano gap by accident through producing a large number of SET canbe merely used in effect. Thus, the method achieves very inferiorproduction yield as a method for production of SET, thereby hamperingthe practical applicability.

The present invention was made for solving these problems in suchconventional methods of the production of SET. Accordingly, an object ofthe present invention is to provide a method for production of SET inwhich a quantum dot is selectively arranged in a nano gap between fineelectrodes, whereby the product yield is significantly improved, leadingto excellent practical applicability.

Means for Solving the Problems

Ferritin is, as shown in FIG. 15, a cage-sharped protein having adiameter of about 12 nm and having a cavity (diameter: about 7 nm)inside thereof formed through binding of 24 subunits. Any of varioustypes of inorganic material particle (core) can be incorporated in thiscavity.

The present inventors found that ferritin specifically adsorbs totitanium in the presence of a nonionic surfactant, and investigated inattempts to selectively fix a quantum dot in the nano gap between fineelectrodes utilizing this property of ferritin. Accordingly, the presentinvention was accomplished.

Specifically, in the method for production of SET of the presentinvention,

the semiconductor element includes a substrate, a source electrode, adrain electrode, and a gate electrode, wherein

a quantum dot is positioned between the source electrode and the drainelectrode,

the substrate has an insulating layer on the top face thereof,

the substrate has the source electrode and the drain electrode, whichare opposed, on the insulating layer,

the source electrode and the drain electrode are provided with atitanium film, and a nontitanium metal film consisting of a metal otherthan titanium that covers the titanium film, respectively,

the method for production including:

a ferritin dropping step for dropping a solution containing ferritinincluding a metal or semiconductor particle therein, thereby allowingthe ferritin to be selectively arranged between the source electrode andthe drain electrode; and

a ferritin decomposing step for decomposing the selectively arrangedferritin, thereby forming the quantum dot consisting of the metal orsemiconductor particle between the source electrode and the drainelectrode.

By thus forming an insulating film on a semiconductor substrate,laminating thereon a titanium film and a nontitanium metal filmconsisting of a metal other than titanium so as to cover the titaniumfilm thereby forming fine electrodes (source electrode and drainelectrode), and dropping thereto a solution containing ferritinincluding a metal or semiconductor particle therein, and a nonionicsurfactant, the ferritin including the metal or semiconductor particletherein is adsorbed preferentially within a nano gap between the fineelectrodes having a particularly high adsorption energy.

Thereafter, upon decomposition of the ferritin, the metal orsemiconductor particle can be fixed as a quantum dot at a centerlocation in the nano gap, therefore, SET can be produced in an efficientmanner.

The gate electrode may be provided below the insulating layer betweenthe source electrode and the drain electrode.

Further, the gate electrode may be provided laterally to the gap betweenthe source electrode and the drain electrode.

Also, the gate electrode may be provided above the gap between thesource electrode and the drain electrode.

In this case, the insulating layer may be a first insulating layer; asecond insulating layer may be provided that covers the top face of thesubstrate having the source electrode, the drain electrode and thequantum dot; and a gate electrode may be provided on the secondinsulating layer.

It is preferred that the nontitanium metal film covering the titaniumfilm be a gold (Au) film.

The nontitanium metal film preferably has a thickness greater than thethickness of the titanium film.

It is preferred that the solution containing ferritin including a metalor semiconductor particle therein further comprises a 0.01% vol/vol ormore and 10% vol/vol or less nonionic surfactant in the ferritindropping step.

Following the ferritin decomposing step, a protection step may beincluded for covering the semiconductor substrate surface on which thesource electrode and the drain electrode were formed with an insulatinglayer for protection.

The foregoing objects, other objects, features and advantages of thepresent invention will be apparent from the detailed description of thefollowing suitable embodiments with reference to the accompanyingdrawings.

ADVANTAGES OF THE INVENTION

According to the method for production of SET of the present invention,by utilizing the selective adsorptivity of ferritin to the material onthe substrate, a quantum dot can be selectively arranged in the nano gapbetween fine electrodes, whereby product yield in the SET production canbe significantly improved.

In addition, the distance between the fine electrode and the quantum dotcan be autonomously regulated by means of the film thickness of theferritin outer shell. Thus, the production process can be simplified,and the gap between the fine electrode and the quantum dot can beutilized as a tunnel barrier.

Moreover, since unwanted quantum dots are not formed around the nano gapbetween the fine electrodes except for the side wall of the electrodes,variation of the element operation can be suppressed.

Furthermore, the tunnel gap distance is specified to be approximately0.5 to 2 nm in the prior arts disclosed in Nonpatent Document 2 andNonpatent Document 3. However, according to the method for production ofSET of the present invention, the tunnel gap distance becomesapproximately 2 to 4 nm, whereby the device operation at a highertemperature can be also contemplated.

Additionally, multiple dot formation which can not be regulatedaccording to the prior arts disclosed in Nonpatent Document 2 andNonpatent Document 3 can be also regulated by adjusting the distancebetween the fine electrodes, whereby enhancement of functionality of thedevice can be also contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show views illustrating one example of the basicstructure of SET having a back gate electrode produced in Embodiment 1,in which FIG. 1A shows a perspective view; FIG. 1B shows a top view; andFIG. 1C shows a cross-sectional view.

FIG. 2 shows a flow chart for demonstrating the method for production ofSET according to Embodiment 1 of the present invention.

FIGS. 3A to 3C show views illustrating the state of arrangement offerritin on the substrate in Embodiment 1 of the present invention, inwhich FIG. 3A shows a perspective view; FIG. 3B shows a top view; andFIG. 3C shows a cross-sectional view.

FIGS. 4A and 4B show views illustrating the state of fixation of thequantum dot around the fine electrodes, in which FIG. 4A shows the stateof fixation according to a conventional method for production; and FIG.4B shows the state of fixation according to Embodiment 1 of the presentinvention.

FIGS. 5A and 5B show cross-sectional views illustrating around the fineelectrodes, in which FIG. 5A illustrates a case of a titanium monolayerelectrode; and FIG. 5B illustrates a case of a bilayer electrode of thisEmbodiment.

FIGS. 6A to 6I show cross-sectional views illustrating the steps of themethod for production of SET of Example 1, respectively.

FIG. 7 shows an electron micrograph for illustrating around the sourceelectrode and the drain electrode of the SET substrate obtained inExample 1.

FIG. 8 shows a graph demonstrating the alteration of drain electriccurrent when the voltage between the source electrode and the drainelectrode was changed on the SET substrate obtained in Example 1.

FIG. 9 shows an electron micrograph for illustrating around the sourceelectrode and the drain electrode of the SET substrate obtained inExample 2.

FIG. 10 shows a graph demonstrating the alteration of source electriccurrent when the voltage between the source electrode and the drainelectrode was changed on the SET substrate obtained in Example 2.

FIGS. 11A and 11B show views illustrating the state of arrangement offerritin in the nano gap between source/drain in Embodiment 1 of thepresent invention, in which FIG. 11A shows a view illustrating the casein which a single quantum dot is formed; and FIG. 11B shows a viewillustrating the case in which double quantum dots are formed.

FIGS. 12A to 12C show views illustrating one example of the basicstructure of SET having a side gate electrode produced in Embodiment 2of the present invention, in which FIG. 12A shows a perspective view;FIG. 12B shows a top view; and FIG. 12C shows a cross-sectional view.

FIGS. 13A to 13C show views illustrating one example of the basicstructure of SET having a top gate electrode produced in Embodiment 3 ofthe present invention, in which FIG. 13A shows a perspective view; FIG.13B shows a top view; and FIG. 13C shows a cross-sectional view.

FIGS. 14A and 14B show views illustrating the state of arrangement of aquantum dot(s) of the fine electrodes and therearound according to aconventional method for production of SET, in which FIG. 14A shows aview illustrating the state of arrangement in which the gap betweenquantum dot/fine electrodes was not regulated; and FIG. 14B shows a viewillustrating the state of arrangement in which unwanted quantum dots arepresent in and around the nano gap between source/drain.

FIG. 15 shows a view illustrating the structure of ferritin.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

1, 21: silicon substrate

2: insulating layer

3: titanium thin film

4: nontitanium metal thin film (metal thin film consisting of a metalother than titanium)

5, 28: source electrode

6, 29: drain electrode

7, 23: back gate electrode

8: quantum dot (metal or semiconductor particle)

9: lateral face quantum dot

10: outer shell protein of ferritin

11: ferritin including a metal or semiconductor particle therein

12: ferritin including a metal or semiconductor particle therein, andadsorbed on fine electrode lateral face

13: selective adsorption region

14: quantum dot

15: unwanted quantum dot

22: silicon oxide film (SiO₂ film)

24: pad electrode

25: electron beam resist

26: electron beam irradiated region

27: vapor deposited Ti thin film and Au thin film

30: indium-including ferritin

31: indium quantum dot

32: wiring with FIB (focused ion beam) apparatus

33: wire

41: side gate electrode

42: first insulating layer

43: second insulating layer

44: top gate electrode

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be explained withreference to the drawing. The present invention is not any how limitedto the following Embodiments.

Embodiment 1

Embodiment 1 illustrates a method for production of SET (single electrontransistor) having a back gate electrode. The basic structure of SETproduced by this Embodiment is shown in FIGS. 1A to 1C.

In this SET, an insulating layer 2 (SiO₂ film) is formed on a siliconsubstrate 1, and thereon are formed a source electrode 5 and a drainelectrode 6 including a laminated titanium thin film 3 and a nontitaniummetal thin film consisting of a metal other than titanium (hereinafter,referred to as “nontitanium metal thin film”) 4 that covers the titaniumthin film 3. The source electrode 5 and the drain electrode 6 are formedso as to oppose one another thereby providing a nano gap therebetween ina plan view. Furthermore, a back gate electrode 7 is formed immediatelybelow the insulating layer 2. The back gate electrode 7 is formed so asto locate below the nano gap and therearound.

At the center location of the nano gap between the source electrode 5and the drain electrode 6 a metal or semiconductor particle isselectively arranged and fixed as a quantum dot 8, and a few unwantedquantum dots can be present around the nano gap.

Although a metal or semiconductor particle (lateral face quantum dot 9)is also present in the vicinity of the lateral face of the sourceelectrode 5 or the drain electrode 6 in SET shown in FIGS. 1A to 1C, ithas less influence on the SET operation.

Next, a method for production of SET of this Embodiment will beexplained.

A flow chart of the production steps of this Embodiment is shown in FIG.2. Referring to FIGS. 1A to 1C and FIG. 2, in step S1, the insulatinglayer 2 is first formed on the silicon substrate 1 on which the backgate electrode 7 had been formed (insulation step). For the formation ofthe insulating layer 2 on the silicon substrate 1, any known method maybe employed, and also type of the insulating layer 2 is not particularlylimited.

Next, in the step S2, an electron beam resist is applied on theinsulating layer 2 formed on the substrate 1, thereby drawing a pair offine electrode patterns with use of an electron beam (drawing step). Thepair of fine electrode patterns correspond to the source electrode 5 andthe drain electrode 6. The pair of fine electrode patterns are drawnsuch that the gap therebetween (hereinafter, gap between the fineelectrodes) is positioned above the back gate electrode 7.

In this step, since one quantum dot is arranged in the gap between thefine electrodes, the drawing is regulated such that the gap between thefine electrodes satisfies within the formula of: minimum electrodeinterval≦ferritin diameter. Such a method of drawing the fine electrodepattern is well known in the field of semiconductors, therefore, thedescription of the same is omitted herein.

Next, in the step S3, the electron beam resist is developed, whereby thefine electrode pattern is formed on the substrate 1 (patterning step).By the development, the electron beam-irradiated region is decomposed offrom the electron beam resist applied on the insulating layer 2, andthus the fine electrode pattern is formed which consists of the exposedregion of the insulating layer 2 on the substrate 1.

Next, in the step S4, the titanium thin film 3, and the nontitaniummetal thin film 4 that covers the titanium thin film 3 are vapordeposited on the substrate 1 on which the fine electrode pattern wasformed in the step S3, whereby two or more layers (herein, two layers)are sequentially laminated (laminating step). The titanium thin film 3corresponding to the underlayer has a thickness of preferably 1 nm orgreater and 12 nm or less. When the thickness is less than 1 nm, contactof the outer shell protein of ferritin with the titanium thin film 3 maybe insufficient. In contrast, when the thickness is beyond 12 nm,adsorption of plural number of the particles of ferritin in theelevational direction can not be suppressed.

In addition, the thickness of the titanium thin film 3 being less than 6nm is more preferred since ferritin adsorbed to the titanium thin film 3also adsorbs to the insulating layer 2 without fail. Failure ofadsorption of ferritin to the insulating layer 2 can lead to contact ofthe core with the source electrode or the drain electrode upondecomposition of the outer shell protein.

Meanwhile, a region opposing to the quantum dot of the titanium thinfilm 3 is oxidized by exposure to the ambient air and the like, therebyresulting in formation of TiO₂. Thus, transfer of the charge between thedot and the source/drain electrodes is more readily executed on thenontitanium metal thin film 4 as compared with that on the titanium thinfilm 3. The thickness of the titanium thin film 3 of equal to or lessthan 3 nm is more preferred because positional shift between thenontitanium metal thin film 4 and the quantum dot in the elevationaldirection can be prevented, whereby charge transfer between thenontitanium metal thin film 4 and the quantum dot is facilitated.

Mode of covering of the titanium thin film 3 by the nontitanium metalthin film 4 will be explained in detail below. As shown in FIG. 1A, thenontitanium metal thin film 4 covers the top face of the titanium thinfilm 3, however, the lateral face of the titanium thin film 3 is notfundamentally covered. However, the lateral face of the titanium thinfilm 3 may be covered by the nontitanium metal thin film 4 except for apart sandwiched between the source electrode 5 and the drain electrode 6(the part in which the quantum dot 8 is present in FIGS. 1A to 1C). Inother words, the part where the lateral face quantum dot 9 is present onthe lateral face of the titanium thin film 3 in FIGS. 1A to 1C may becovered by the nontitanium metal thin film 4.

The part sandwiched between the source electrode 5 and the drainelectrode 6 (the part in which the quantum dot 8 is present in FIGS. 1Ato 1C) must not be covered by the nontitanium metal thin film 4. Whenthis part is covered, the quantum dot 8 sandwiched between the sourceelectrode 5 and the drain electrode 6 is not formed.

Examples of the metal other than titanium which may be preferably usedinclude noble metals such as gold (Au), platinum (Pt), silver (Ag),palladium (Pd) and the like. The nontitanium metal thin film 4 of thesecond or latter layer(s) has a thickness of preferably 2 nm or greaterand 100 nm or less. When the thickness is less than 2 nm, resistance ofthe nontitanium metal thin film 4 may be great, and in contrast, whenthe thickness is beyond 100 nm, production of the nano gap electrode bylifting off or the like may be difficult. Also, as long as the outermostlayer of the fine electrode is the nontitanium metal thin film, thenontitanium metal thin film may either have one layer, or have two ormore layers or the same of different metal(s).

Next, in the step S5, the substrate following the laminating step isdipped in an organic solvent, and the electron beam resist below thetitanium thin film is lifted off, whereby the titanium thin film and thenontitanium metal thin film are formed as a fine electrode (electrodeformation step). When the electron beam resist is lifted off, the sourceelectrode 5 and the drain electrode 6 each having the under layer beingthe titanium thin film 3 and the upper layer being the nontitanium metalthin film 4 are formed on the insulating layer 2. Accordingly, the nanogap between source/drain would have the interval adjusted in the drawingstep.

Next, in the step S6, a solution containing ferritin including a metalor semiconductor particle therein, and a nonionic surfactant isprepared, and this solution is dropped on the substrate 1 following theelectrode formation step. Thus, the ferritin including the metal orsemiconductor particle therein is selectively arranged in the nano gapbetween source/drain (not shown in the Figure: ferritin dropping step).

The nonionic surfactant is not particularly limited, but Tween 20 andTween 80 were used in Examples described later. Also, the concentrationof the nonionic surfactant is preferably 0.01% vol/vol or greater and10% vol/vol or less.

The metal or semiconductor particle included in ferritin as the core isnot particularly limited. The process for preparing ferritin includingthe metal or semiconductor particle therein will be described later byway of Examples.

After dropping the solution containing ferritin including the metal orsemiconductor particle therein, and a nonionic surfactant on thesubstrate 1 following the electrode formation step, the substrate 1 iswashed using a wash fluid. This washing results in elimination offerritin other than the ferritin including the metal or semiconductorparticle therein which was either selectively adsorbed and arranged inthe nano gap between source/drain, or adsorbed on titanium of thelateral face of the source electrode 5 and drain electrode 6, from thesubstrate 1.

Next, in the step S7, the outer shell protein of the ferritin includingthe metal or semiconductor particle therein on the substrate 1 isdecomposed (ferritin decomposing step). The process for thedecomposition may be by, for example, means such as heating, ultravioletirradiation, ozone oxidation and the like, which may be alone oremployed in combination. Upon decomposition of the ferritin, the metalor semiconductor particle (core) which had been included therein isfixed on the insulating layer 2 as the quantum dot 8 at the placeunchanged where the ferritin including the metal or semiconductorparticle therein had been arranged.

Still more, following the step S7, similarly to common semiconductordevices, the substrate 1 top face is covered with the insulating layerfor protection (protection step). For covering the substrate 1 top face,known method may be utilized.

Next, the method for production of SET of this Embodiment will beexplained.

According to this Embodiment, in the step S5, the fine electrodes(source electrode 5 and drain electrode 6) are formed on the siliconsubstrate 1 on which the insulating layer 2 (for example, SiO₂ film) hasbeen formed on its top face by sequentially laminating the titanium thinfilm 3 and the nontitanium metal thin film 4, as shown in FIGS. 3A to3C.

In this step, the nano gap between the source electrode 5 and the drainelectrode 6 (hereinafter, nano gap between source/drain) is adjusted soas to satisfy the formula of minimum electrode interval≦ferritindiameter. Although the insulating layer 2 is provided on the top face ofthe substrate 1 in this Embodiment, the entire substrate may be theinsulator such as the case of, for example, an organic film because thesubstrate shall have the insulating layer on the top face also in such acase. However, the substrate is typically composed of a semiconductor,and the insulating layer is formed on the top face thereof by oxidation.

When a solution containing ferritin 11 including a metal orsemiconductor particle 8 therein, and a nonionic surfactant is droppedon such a substrate 1 in the step S6, the ferritin 11 including themetal or semiconductor particle 8 (core) therein is selectively adsorbedon titanium of the lateral face of the source electrode 5 and drainelectrode 6.

In contrast, it is not adsorbed on the part other than titanium on thesubstrate 1, i.e., top face of the fine electrode (nontitanium metalthin film 4) and on the insulating layer 2 (see, FIGS. 3A and 3B).

Particularly, in the nano gap between source/drain, the ferritin 11including the metal or semiconductor particle 8 therein is adsorbed onthe titanium part of the lateral faces of both the source electrode 5and the drain electrode 6, therefore, adsorption energy is two timesgreater as compared with the ferritin (ferritin including a metal orsemiconductor particle therein, and adsorbed on fine electrode lateralface) 12 shown in FIGS. 3A and 3B (area of the selective adsorptionregion 13 being two-fold). Accordingly, the ferritin 11 including themetal or semiconductor particle 8 therein is preferentially adsorbed andarranged in the nano gap between source/drain.

When the substrate is washed, the ferritin 11 including the metal orsemiconductor particle 8 therein that is present on the nontitaniummetal thin film 4 or the insulating layer 2 is eliminated from thesubstrate together with the wash fluid. Then, only the ferritinincluding the metal or semiconductor particle 8 therein (ferritin 11 andferritin 12 shown in FIGS. 3A and 3B) adsorbed on titanium on thelateral faces of the source electrode 5 and the drain electrode 6 isleft on the substrate. Particularly, the ferritin 11 is less likely tobe eliminated in washing because of greater adsorption energy than thatof the ferritin 12.

Next, when the outer shell protein 10 of the ferritin 11 is decomposedby heating or the like of the substrate following the adsorption offerritin in the step S7, the metal or semiconductor particle 8 which hadbeen included is fixed at the center location of the nano gap betweensource/drain as the quantum dot 8. Thus, the gap between the quantum dotand the fine electrode (hereinafter, gap between quantum dot/fineelectrodes) can be utilized as a tunnel barrier.

Although unnecessary quantum dot is not present in and around the nanogap between source/drain except for the side wall of the electrode, thequantum dot is fixed also at the position where the ferritin 12 had beenadsorbed and arranges as shown in FIGS. 3A and 3B (lateral face of thesource electrode 5 or the drain electrode 6). However, because many ofthese quantum dots (for example, quantum dot 9 shown in FIG. 1B) arepositioned away from the gap between the source/drain, the charge isless likely to be trapped, and even though it happens to be trapped,less influence will be imparted on the operation point of SET.

Hereinafter, this action and effect will be explained in more detail.

<Autonomous Control of Gap between Quantum Dot/Fine Electrodes>

In general, the nano gap between source/drain varies every individualfine electrodes. Also in one fine electrode, as shown in FIGS. 4A and4B, there exist differences (variation) in size of the gap depending onthe position.

As described above, in conventional methods for production of SETdisclosed in Nonpatent Document 2 and Nonpatent Document 3, even thoughthe quantum dot 14 is fixed in the nano gap between source/drain asshown in FIG. 4A, control of the gap between quantum dot/fine electrodesis impossible, and thus the quantum dot could not be intentionally fixedat the center location of the nano gap between source/drain.

To the contrary, in the ferritin arrangement step of this Embodiment,ferritin is preferentially adsorbed and arranged at the center locationof the nano gap between source/drain in the presence of the nonionicsurfactant, however, a position where the potential energy is minimized(position where adsorbed without aberration from lateral faces of boththe source electrode and the drain electrode) is selected andtransferred on each fine electrode.

As a consequence, when the variation of the nano gap betweensource/drain falls within a suitable range, the metal or semiconductorparticle 8 (core) included in ferritin is autonomously fixed at centerof the nano gap between source/drain in the state of keeping the quantumdot/fine electrode gap determined depending on the thickness (filmthickness) of the ferritin that forms the outer shell, as shown in FIG.4B.

Accordingly, the quantum dot can be selectively fixed at the mostsuitable position in the nano gap between source/drain in thisEmbodiment, therefore, the SET product yield can be significantlyimproved as compared with the conventional methods for production of SETwhich are dependent on accidental event.

<Suppression of Crosslinking Adsorption>

It would be sufficient to constitute the source electrode and the drainelectrode with titanium alone when adsorption and arrangement offerritin in the nano gap between source/drain shall be merely effected.However, in this case, “crosslinking adsorption” of the nano gap betweensource/drain as shown in FIG. 5A may be caused at a part that is smallerthan the external diameter of ferritin 11 including a metal orsemiconductor particle 8 (core) therein.

When the outer shell protein 10 is decomposed in this state, theposition where the metal or semiconductor particle 8 (core) which hadbeen included therein is fixed may be altered due to the adsorptionangle attained between the source electrode 5 and drain electrode 6, andthe ferritin 11 in the state of crosslinking adsorption. In the exampleshown in FIG. 5A, the gap between quantum dot/fine electrodes (a1 anda2) has been smaller than the film thickness of the outer shell protein10.

Also, after the decomposition of the outer shell protein 10, the quantumdot 8 may be contacted with the surface of the source electrode 5 ordrain electrode 6 in some cases. Under such circumstances, it would beimpossible to utilize the gap between quantum dot/fine electrodes as atunnel barrier.

Thus, in this Embodiment, the source electrode 5 and the drain electrode6 are formed through covering the titanium thin film 3 with thenontitanium metal thin film 4 (laminating the nontitanium metal thinfilm 4 on the titanium thin film 3) so as to provide the outermost layerthat is a nontitanium metal as shown in FIG. 5B. By thus constructing tohave such a structure, “crosslinking adsorption” of the ferritin 11including the metal or semiconductor particle 8 (core) therein betweenthe source electrode 5 and the drain electrode 6 can be prevented in theferritin arrangement step (step S6). As a consequence, after thedecomposition of the outer shell protein 10, the gap between the quantumdot/fine electrodes (a3) and (a4) becomes almost equal to the filmthickness of the outer shell protein 10.

In Prior Arts, the tunnel gap distance has been set to be approximately0.5 to 2 nm, and the film thickness of the outer shell protein 10 hasbeen about 2.5 nm. The gap between quantum dot/fine electrodes may bealso dependent on the core size and shape, it can be adjusted toapproximately 2 to 4 nm. Therefore, attempts of elevation of thetemperature of the device operation can be made in this Embodiment.

EXAMPLE 1

Next, in Example 1, a source electrode and a drain electrode constitutedwith a titanium thin film and a gold (Au) thin film were formed on thesilicon substrate, and SET in which an indium quantum dot wasselectively fixed in the nano gap between source/drain was produced.Hereinafter, with reference to FIGS. 6A to 6I, Example 1 will beexplained.

(Insulation Step)

First, a back gate electrode 23 was produced on a silicon substrate 21.Then, a SiO₂ film 22 was formed as the insulating layer on the top faceof the silicon substrate 21. In addition, on the SiO₂ film 22 was formeda pad electrode 24 for the wiring to be perfected later. Moreover, afterwashing the top face of the silicon substrate 1 with pure water, it wascleaned by UV (ultraviolet) irradiation in the presence of ozone (O₃) at110° C. for 10 min using an UV/ozonation apparatus (see, FIG. 6A).

(Drawing Step)

Next, the silicon substrate 21 was placed in a spin coater, and theretowas added dropwise a solution prepared by adding anisole to an electronbeam resist (Zeon Corporation, ZEP520A) thereby diluting to give aconcentration of 25%. Then, after spinning the silicon substrate 1 at2000 rpm for 5 sec, it was further spun at 4000 rpm for 60 sec.Thereafter, the substrate 1 was prebaked on a hot plate at 140° C., for3 min, where fixation of an electron beam resist 25 was permitted (see,FIG. 6B).

After cooling to room temperature, a fine electrode pattern was drawnusing an electron beam bean exposing apparatus so as to give the nanogap between source/drain of 20 nm (see, FIG. 6C).

(Patterning Step)

Subsequently, the silicon substrate 21 on which the fine electrodepattern was drawn was immersed in n-amyl acetate for 1 min. Thereafter,excess n-amyl acetate was removed with nitrogen gas blow to produce afine resist pattern on the silicon substrate 21 (see, FIG. 6D).

(Laminating Step)

Next, the silicon substrate 21 on which the fine resist pattern wasproduced was placed in a vapor deposition apparatus, and vacuum pumpingwas carried out. Then, vapor deposition of a titanium (Ti) thin film wasfirst conducted to give a thickness of 2 nm. Thereafter, vapordeposition of a gold (Au) thin film was carried out to give a thicknessof 10 nm, whereby a gold thin film was laminated (reference sign 27)such that the titanium thin film is covered (see, FIG. 6E).

(Electrode Formation Step)

Next, the silicon substrate 21 on which the titanium thin film and thegold thin film were laminated was immersed for 10 min in dimethylacetamide which had been kept at 40° C. Thereafter, the substrate in avessel involving dimethyl acetamide was placed as a whole in anultrasonic cleaning apparatus, and subjected to ultrasonic cleaning for5 min. Following the ultrasonic cleaning, the silicon substrate 21 wasremoved, and the surface was rinsed with acetone.

Furthermore, after rinsing the substrate surface with pure water, thesilicon substrate 21 was placed in a spin coater, and spun at 2000 rpmfor 5 sec, and thereafter spun at 4000 rpm for 30 sec to eliminateexcess moisture. By the steps thus far, the titanium thin film and thegold thin film were laminated to form a source electrode 28 and a drainelectrode 29 (nano gap between source/drain=20 nm) (see, FIG. 6F).

(Ferritin Dropping Step)

Next, a solution containing ferritin including an indium particletherein (indium-including ferritin 30), and a nonionic surfactant wasprepared. Preparation of the ferritin 30 including the indium particletherein will be explained in detail below.

Although naturally occurring ferritin (derived from equine spleen) iscomposed of 24 subunits assembled together, the naturally occurringferritin does not have a constant structure since there are two types ofthe subunit having slightly distinct structures, i.e., L type and Htype. Therefore, recombinant ferritin composed of the L type subunitalone was used in this Example.

First, a DNA encoding L type ferritin (SEQ ID NO: 1, 507 base pairs) wasamplified using a PCR technique to provide a large amount of L typeferritin DNA.

Subsequently, this L type ferritin DNA was cleaved at sites which arespecifically cleaved by restriction enzymes EcoRI and Hind III(restriction enzyme sites). By this treatment of cleavage, a solutioncontaining the L type ferritin DNA fragment having the EcoRI and HindIII restriction enzyme sites was prepared. DNA electrophoresis of thissolution was carried out, whereby only the DNA fragment encoding the Ltype ferritin was recovered and purified.

Thereafter, this L type ferritin DNA fragment was incubated with avector plasmid (pMK-2) which had been treated with the EcoRI-Hind IIIrestriction enzymes to effect ligation. Thus, a vector plasmidpMK-2-fer-8 in which the L type ferritin DNA was incorporated in thepMK-2 plasmid at a multicloning site (MCS) was produced. The employedvector plasmid pMK-2 was selected because it is advantageous inobtaining a large amount of ferritin, which is characterized by being amulticopy plasmid having Tac promoter as a promoter, thereby yieldingmany number of copies. Thus produced plasmid (pMK-2-fer-8) wasintroduced (transformed) into E. coli Nova Blue (Novagen, Escherichiacoli strain) that is a host to produce a recombinant L type ferritinstrain (fer-8).

The fer-8 strain was recovered by centrifugal separation at a low speed,and suspended in a 50 mM Tris-HCl buffer (pH 8.0, +150 mM NaCl). Aftersubjecting this solution to ultrasonic vibration at 60° C. for 20 min,the host cells were recovered again by centrifugal separation at a lowspeed. Recombinant ferritin (apoferritin) in the suspension was purifiedby an ion exchange column (Q-sepharose, Amarsham Biosciences) and gelfiltration (Hiprep Sephacryl S-300: Amarsham Biosciences; and G4000SWXLPEEK: Tosoh Corporation).

Eluate from the ion exchange column was fractionated, and identified onSDS-PAGE. A fraction containing the recombinant ferritin (apoferritin,SEQ ID NO: 2) alone was recovered, and subjected to gel filtration.Accordingly, a recombinant ferritin monomer was recovered.

Next, a solution of 200 mM monosodium phosphate, 40 mM hydrochloric acidand 4 mM ammonia was prepared, and the pH was adjusted to about 2.8.Using this solution, a recombinant ferritin (fer-8) solution wasprepared to give the concentration of 0.1 mg/mL, and thereto was furtheradded 20 mM indium sulfate so as to give the final concentration of 1mM. The reaction solution was stirred, and thereafter left to standovernight.

Subsequently, the recombinant ferritin including the indium compoundcore formed therein was purified and recovered the molecules from thesolution following the reaction by centrifugal separation and gelfiltration. The centrifugal separation was carried out under a conditionof 4,000 G for 30 min, and unnecessary portion other than ferritin waseliminated stepwise in the form of precipitates. The recombinantferritin including the indium core formed therein was concentrated fromthe finally left supernatant by centrifugal concentration using acentrifuge filter (450 nm, Centriprep 50, manufactured by Amicon).

Free indium and aggregation of ferritin molecules were eliminated fromthus resulting recombinant ferritin (indium-including ferritin) usingcolumn chromatography (Sephadex G-25 and Cephacryl S-300 column).

Finally, the recombinant ferritin was concentrated by centrifugalconcentration using a centrifuge filter (450 nm, Centriprep 50,manufactured by Amicon) to give the concentration of 3.0 mg/mL.

To thus resulting ferritin including the indium compound therein(indium-including ferritin 30) was added a MES/Tris buffer (100 mM, pH7.0) containing 1% vol/vol of Tween 20 added as a nonionic surfactant toadjust the ferritin concentration of 2 mg/mL. The ferritin solutionafter adjusting the concentration was dropped on the substrate on whichthe fine electrodes were formed, and kept for 30 min, whereby theindium-including ferritin 30 was selectively adsorbed and arranged inthe nano gap between source/drain.

Thirty minutes later, the silicon substrate 21 was rinsed using purewater, and washed with running pure water for 5 min. After washing, thesubstrate 1 was placed in a spin coater, which was spun at 2000 rpm for5 sec, and thereafter spun at 4000 rpm for 30 sec to eliminate excessmoisture (see, FIG. 6G).

(Ferritin Decomposing Step)

Next, using an UV/ozonation apparatus, the outer shell protein offerritin on the silicon substrate 21 was decomposed by irradiating UV(ultraviolet) in the presence of ozone (O₃) at 110° C. for 40 min to fixthe indium compound included in ferritin as the quantum dot 31 (see,FIG. 6H).

An electron micrograph of the source electrode 28 and the drainelectrode 29, and the vicinity thereof on the silicon substrate 21following the ferritin decomposing step is shown in FIG. 7.

In FIG. 7, the indium quantum dot has been fixed on the lateral face ofthe source electrode 28 on the left handside of the image, and of thedrain electrode 29 on the right handside of the image, but no indiumquantum dot was present except for the side wall of the electrode in andaround the nano gap between source/drain (about 20 nm).

Next, as shown in FIG. 6I, wirings 32 were provided on the pad electrode24, the source electrode 28 and the drain electrode 29 on the substratefollowing the ferritin decomposing step for output of the signalutilizing an FIB (focused ion beam) apparatus (see, FIG. 6I). Thesilicon substrate 21 after the wiring was placed in a low temperatureprober (PPMS, manufactured by Quantum Design Japan Inc.), and thetemperature was set to be 4.2 K (Kelvin). Thereafter, for determinationof the electric characteristics, the output terminal was connected to anapparatus for determining semiconductor parameters (4156C, manufacturedby Agilent Technologies Japan, Ltd.).

First, setting of the gate electrode was made at 0 V, and the voltagebetween the source electrode and the drain electrode was changed in therange of from −0.6 V to +0.6 V, whereby alteration of the drain electriccurrent was measured. The measurement results are shown in FIG. 8.

As is clear from FIG. 8, stepwise electric current-voltagecharacteristics were achieved, and probability of the substrate ofExample 1 bearing a coulomb blockade effect was found. In other words,it was ascertained that the substrate of Example 1 functioned as SET.Moreover, it was also verified that the source electrode and the drainelectrode were insulated in the element without any dot produced in asimilar manner.

Although Tween 20 was used as a nonionic surfactant in Example 1,similar electric current-voltage characteristics were verified also onthe SET substrate produced with use of Tween 80 at the sameconcentration.

Modification Example of this Embodiment

In this Embodiment and Example 1, the gap between the fine electrodeswas adjusted so as to satisfy the formula of: minimum electrodeinterval≦ferritin diameter for arranging one quantum dot in the gapbetween the fine electrodes. When arrangement of two quantum dots in thegap between the fine electrodes is intended, the gap may be adjusted soas to satisfy the formula of: ferritin diameter≦minimum electrodeinterval<ferritin diameter×2.

More specifically, when the minimum interval between the sourceelectrode and the drain electrode is smaller than the diameter offerritin of about 12 nm (i.e., in the case of satisfying the formula of:minimum electrode interval≦ferritin diameter), the ferritin 11 includingthe metal or semiconductor particle 8 (core) therein is selectivelyadsorbed at a position where the potential energy is minimum between thesource electrode 5 and the drain electrode 6 as shown in FIG. 11A. Inthis case, after the decomposition of the outer shell protein 10, onequantum dot (single dot) would be formed at the center location of thenano gap between source electrode 5/drain electrode 6.

On the other hand, when the minimum interval between the sourceelectrode 5 and the drain electrode 6 is greater than the diameter offerritin and smaller than twice the diameter of ferritin (i.e., in thecase of satisfying the formula of: ferritin diameter<minimum electrodeinterval≦ferritin diameter×2), each one ferritin 11 including the metalor semiconductor particle 8 (core) therein is adsorbed on titanium ofthe lateral face of the source electrode 5 and the drain electrode 6,respectively, and the ferritin 11 would adsorb one another by theadsorption between proteins, as shown in FIG. 11B.

These two particles of the ferritin 11 are rigidly fixed by the bindingforce of the lateral face of the source electrode 5 or the drainelectrode 6 with titanium, and interprotein binding force of theferritin 11 one another, therefore, even though the substrate is washed,they remain at the position shown in FIG. 11B. Then, after thedecomposition of the outer shell protein 10, two quantum dots (doubledot) would be formed in the nano gap between source electrode 5/drainelectrode 6.

Accordingly, in the modified example of this Embodiment, multiple dotformation between the fine electrodes can be controlled by adjusting thegap between the fine electrodes whereby enhancement of functions of SETcan be realized although such control had been conventionallyimpossible.

EXAMPLE 2

In Example 2, a source electrode and a drain electrode constituted witha titanium thin film and a gold (Au) thin film were formed on thesilicon substrate, and SET in which a cobalt quantum dot was selectivelyfixed in the nano gap between source/drain was produced. Example 2 isdifferent from Example 1 merely in terms of the metal included inrecombinant ferritin (fer-8) being cobalt, therefore, only a method ofpreparing ferritin including cobalt therein will be explained.

A solution of 0.5 mg/mL (1 μM) recombinant ferritin/100 mM Tris-HCl (pH7.3-8.8) was prepared, and thereto was added 37.5 mM ammonium cobaltsulfate. While stirring the reaction solution with a magnetic stirrer,ammonium cobalt sulfate was added to give the final concentration of 2to 5 mM, and further, an aqueous hydrogen peroxide solution was addedthereto in an amount half the stoichiometric number of the ammoniumcobalt sulfate. The reaction solution was stirred for 20 min, andthereafter, the temperature of the reaction solution was kept at 50° C.,which was left to stand overnight.

The recombinant ferritin including the cobalt core formed therein waspurified and recovered from the solution following the reaction bycentrifugal separation and gel filtration. The centrifugal separationwas carried out under a condition of 1,600 G for 10 min and 10,000 G for30 min, and unnecessary portion other than ferritin was eliminatedstepwise in the form of precipitates. The recombinant ferritin includingthe cobalt core formed therein was recovered in a pellet byultracentrifugal separation at 230,000 G, for 1 hour from the finallyyielded supernatant.

Thus resulting recombinant ferritin was subjected to gel filtration(column: TSK-GEL G4000 SWXL, PEEK/flow rate: 1 mL/min/buffer: 50 mMTris-HCl (pH 8.0)+150 mM NaCl) using HPLC, and a peak of 24-mer (about480 kDa) was fractionated. Thus fractionated recombinant ferritinsolution was concentrated using an ultrafiltration membrane to obtainrecombinant ferritin including a cobalt particle (Co₃O₄) therein(cobalt-including ferritin).

Using thus prepared cobalt-including ferritin, SET substrate wasproduced in a similar manner to Example 1.

An electron micrograph of the source electrode 28 and the drainelectrode 29, and the vicinity thereof on the silicon substrate 1following the ferritin decomposing step of Example 2 is shown in FIG. 9.

In FIG. 9, the cobalt quantum dot has been fixed on the lateral face ofthe source electrode on the left handside of the image, and of the drainelectrode on the right handside of the image, but no cobalt quantum dotwas present except for the side wall of the electrode in and around thenano gap between source/drain (about 20 nm).

Next, similarly to Example 1, setting of the gate electrode was made at0 V, and the voltage between the source electrode and the drainelectrode was changed in the range of from −0.6 V to +0.6 V, wherebyalteration of the source electric current was measured. The measurementresults are shown in FIG. 10.

As is clear from FIG. 10, stepwise electric current-voltagecharacteristics were achieved, and probability of the substrate ofExample 2 bearing a coulomb blockade effect was found. Thus, it wasascertained that the substrate of Example 2 functioned as SET. Moreover,it was also verified that the source electrode and the drain electrodewere insulated in the element without any dot produced in a similarmanner.

Although Tween 20 was used as a nonionic surfactant in Example 2,similar electric current-voltage characteristics were verified also onthe SET substrate produced with use of Tween 80 at the sameconcentration.

Embodiment 2

Embodiment 2 of the present invention illustrates a method forproduction of SET having a side gate electrode. Basic structure of SETproduced by this Embodiment is shown in FIGS. 12A to 12C.

In this SET, a side gate electrode 41 constituted with a material otherthan titanium is formed on the lateral side of the nano gap between thesource electrode 5 and the drain electrode 6. Except for thisconstitution, this SET has the same structure as that of SET shown inFIGS. 1A to 1C. It is preferred that the side gate electrode 41 beprovided away from the source electrode 5 and the drain electrode 6farther than the distance of the nano gap between the source electrode 5and the drain electrode 6.

A metal or semiconductor particle is selectively arranged and fixed asthe quantum dot 8 at a center location of the nano gap between thesource electrode 5 and the drain electrode 6, and unnecessary quantumdot is not present around the nano gap except for the side wall of theelectrode.

In SET shown in FIGS. 12A to 12C, the metal or semiconductor particle(lateral face quantum dot 9) is present also in the vicinity of thelateral face of the source electrode 5 or the drain electrode 6.However, because many of these are positioned away from the gap betweensource/drain, the charge is less likely to be trapped, and even thoughit happens to be trapped, less influence will be imparted on theoperation point of SET.

SET of this Embodiment can be produced by a method for productionsimilar to that of Embodiment 1. Moreover, modified examples similar toEmbodiment 1 having electric characteristics similar to SETs of Example1 and Example 2 may be also produced.

Embodiment 3

Embodiment 3 of the present invention illustrates a method forproduction of SET having a top gate electrode. Basic structure of SETproduced by this Embodiment is shown in FIGS. 13A to 13C.

In this SET, the source electrode 5 and the drain electrode 6 on thefirst insulating layer 42 are covered by a second insulating layer 43,and on this second insulating layer 43 is formed a top gate electrode44. The top gate electrode 44 is formed so as to be positioned above thenano gap between the source electrode 5 and the drain electrode 6, andtherearound. Except for this constitution, this SET has the samestructure as that of SET shown in FIGS. 1A to 1C.

A metal or semiconductor particle is selectively arranged and fixed asthe quantum dot 8 at a center location of the nano gap between thesource electrode 5 and the drain electrode 6, and unnecessary quantumdot is not present around the nano gap except for the side wall of theelectrode.

Also in SET shown in FIGS. 13A to 13C, the metal or semiconductorparticle (lateral face quantum dot 9) is present also in the vicinity ofthe lateral face of the source electrode 5 or the drain electrode 6.However, because many of these are positioned away from the gap betweensource/drain, the charge is less likely to be trapped, and even thoughit happens to be trapped, less influence will be imparted on theoperation point of SET.

SET of this Embodiment can be produced by a method for productionsimilar to that of Embodiment 1. Moreover, modified examples similar toEmbodiment 1 having electric characteristics similar to SETs of Example1 and Example 2 may be also produced.

From the foregoing description, many improvements and other embodimentsof the present invention will be apparent to persons skilled in the art.Therefore, the foregoing description should be merely construed asillustration, which is provided for the purpose of teaching personsskilled in the art the best mode for carrying out the present invention.Details of the structure and/or function of the present invention can besubstantially altered without departing from the spirit of theinvention.

INDUSTRIAL APPLICABILITY

The method for production of a single electron semiconductor element ofthe present invention is useful as a method for production and the likeof a single electron semiconductor element capable of conveniently andefficiently producing SET in which a quantum dot is selectively arrangedin a nano gap between the fine electrodes.

1. A method for production of a single electron semiconductor element,the semiconductor element comprising a substrate, a source electrode, adrain electrode, and a gate electrode, wherein a quantum dot ispositioned between the source electrode and the drain electrode, thesubstrate has an insulating layer on the top face thereof, the substratehas the source electrode and the drain electrode, which are opposed, onthe insulating layer, the source electrode and the drain electrode areprovided with a titanium film, and a nontitanium metal film consistingof a metal other than titanium that covers the titanium film,respectively, said method for production comprising: a ferritin droppingstep for dropping a solution containing ferritin including a metal orsemiconductor particle therein, thereby allowing the ferritin to beselectively arranged between the source electrode and the drainelectrode; and a ferritin decomposing step for decomposing theselectively arranged ferritin, thereby forming the quantum dotconsisting of the metal or semiconductor particle between the sourceelectrode and the drain electrode.
 2. The method for production of asingle electron semiconductor element according to claim 1 wherein thegate electrode is provided below the insulating layer between the sourceelectrode and the drain electrode.
 3. The method for production of asingle electron semiconductor element according to claim 1 wherein thegate electrode is provided laterally to the gap between the sourceelectrode and the drain electrode.
 4. The method for production of asingle electron semiconductor element according to claim 1 wherein thegate electrode is provided above the gap between the source electrodeand the drain electrode.
 5. The method for production of a singleelectron semiconductor element according to claim 4 wherein theinsulating layer is a first insulating layer; a second insulating layeris provided that covers the top face of the substrate having the sourceelectrode, the drain electrode and the quantum dot; and a gate electrodeis provided on the second insulating layer.
 6. The method for productionof a single electron semiconductor element according to claim 1 whereinthe metal other than titanium is gold.
 7. The method for production of asingle electron semiconductor element according to claim 1 wherein thenontitanium metal film has a thickness greater than the thickness of thetitanium film.
 8. The method for production of a single electronsemiconductor element according to claim 1 wherein the solutioncontaining ferritin including a metal or semiconductor particle thereinfurther comprises a 0.01% vol/vol or more and 10% vol/vol or lessnonionic surfactant in the ferritin dropping step.
 9. The method forproduction of a single electron semiconductor element according to claim1 which further comprises following the ferritin decomposing step aprotection step for covering the semiconductor substrate surface onwhich the source electrode and the drain electrode were formed with aninsulating layer for protection.